Muffle furnace

“ This furnace acts very well, and has been widely introduced both in Great Britain and in other countries, but it has one great drawback, apart from its high cost, viz. that all the hydrochloric acid gas gets mixed with fire-gases, and consequently is condensed in a weaker and less pure form than from ordinary pots and muffles. This has led some factories which had introduced such furnaces to revert to hand-wrought muffle-furnaces. ”

“ A reverberatory furnace is used, in which is a stirring apparatus, so that the heated mass may be continually turned over to accelerate the oxidation. Muffle furnaces are also used, in which the massicot is placed in crucibles on an iron plate, which can be pulled out of the furnace for observation of the change of colour. Whatever method is used, the temperature must be so regulated that over-heating of the material is avoided, otherwise litharge is formed, which is only very slowly converted into red lead. ”

“ Instead of the furnace above described, a muffle furnace may be employed, in which the heat is applied indirectly to the ore, with the precaution of constructing the sole or bottom of this furnace of a material not rapidly acted on by the constituents of the ore ”

“ If fresh, the muffle or furnace must be left open, so as to permit the mixture to be well and frequently stirred, and additions to be made at intervals until eventually ammonia ceases to be evolved. The furnaces are arranged in such a manner that when the carbonate of potash has once become fused the doors of the fire-place may be shut, and no fresh firing is required during the introduction of the animal matters. The molten mass is kept well stirred by means of a thick iron bar, suspended by a chain, and fixed in an aperture in the side of the furnace. ”

“ The muffle is placed over the fireplace of a rectangular furnace, and the flame plays round the sides in such a way as to ensure the uniform distribution of the heat. For the sake of greater cleanliness and the avoidance of dust, the pieces to be fired are placed upon tiles of porcelain rather than upon biscuit or fireclay supports. The temperature may vary from a dull to a full red heat (600° to 1000°C.) , and the firing lasts from four to twelve hours. ”

“ Chaplet has patented a muffle or tube furnace, similar in principle, for use on a larger scale, with a number of electrodes placed above and below the muffle-tube. The arc furnaces now widely used in the manufacture of calcium carbide on a large scale are chiefly developments of the Siemens furnace. ”

“ The top of the muffle is usually slightly arched from side to side, and it is placed in the furnace on a tolerably thick stone floor, so that the bottom may not get too hot. The fire, which is lighted below, is allowed to play up its sides and over its top, the flue being so built as to draw the flames in that direction, for a top heat is the best heat for firing glass regularly. ”

“ The “Reverberatory furnace” is a modification of the wind furnace, and is used to throw heat on to the surface of substances, as in roasting ores of metals, to drive off the sulphur, arsenic, &c., or in the making of soda, litharge, and other processes where the admission of hot air with the flame is either beneficial, or at least not detrimental; fig. 1 shows the construction of this kind of furnace. Blast furnaces are for the production of the very highest degrees of temperature, and in these the air is forced into the fire by blowing machines or bellows, often worked by steam-engines ”

“ The difficulty is that with furnaces of the “open-fire” type the rise of dust is so great that the gauze screens soon become clogged, especially as the slight tarry matter in the smoke condenses on the gauze, causing the dust to adhere. With the better types of furnaces, the employment of gauze screens is not necessary, as there should be very little rise of dust. It is sufficient to use only expanded metal, to prevent any displaced pieces of rubber falling into the furnace chamber. ”

“ These are known as muffle or chamber furnaces; and by supposing the crucibles or retorts to represent similar chambers of only temporary duration, the ordinary pot melting air furnaces, and those for the reduction of zinc ores or the manufacture of coal gas, may be included in the same category. These are almost invariably air furnaces, though sometimes air under pressure is used, as, for example, in the combustion of small anthracitic coal, where a current of air from a fan-blower is sometimes blown under the grate to promote combustion. ”

“ The heat that the worker about these furnaces must face may be divided in general into two classes: there is first that heat due to the flame and hot gases that the blast in the furnaces forces out onto a man's body and face. In the majority of furnaces Page 193 this is by far the most discomforting, and care must be taken to fend it and turn it behind a suitable shield. The second class is the radiant heat, discharged as light from the glowing interior of the furnace. ”

“ In the melting furnaces the gas is admitted at each end of the furnace and mixes with air which has previously been heated by passing through flues in the brickwork. The combustion thus takes place in the melting chamber directly above the crucible pots, and produces an intense and easily regulated heat. The blowing furnaces are even simpler. They merely provide a chamber of brickwork with suitable openings in the sides, and immediately under each opening a large Bunsen burner supplied with natural gas and drawing the requisite air directly from the atmosphere. ”

“ It can be applied in a working pit, and is certain and steady in its operation. The only defect of the furnace is, its liability to explosion when the current of air by which it is supplied, becomes so charged with inflammable air, in its passage through the workings, as to be explosive. This objection to the furnace is, however, greatly obviated by what I call the system of double ventilation. The principle of this system is to supply the furnace with a current of air, which is not exposed to the hazard of being contaminated by an admixture of inflammable air. ”

“ The first-helper gives her that, passing up and down every few minutes to look through the peepholes in her furnace-doors. He puts his glasses down on his nose, inspects the brew, and notices if her stomach's in good shape. If the bricks get as red as the gas flame, she's burning the living lining out of her. But he keeps the gas blowing in her ends, as hot as she'll stand it without a holler. On [Pg 34] either end the gas, and on top of it the air. The first-helper, who is cook of the furnace, makes a proper mixture out of them. ”

“ A blast-furnace, once lit, remains burning till the end of its existence. I got inside her, and was delighted to satisfy a deep-seated curiosity: we crawled in the cinder notch. The hearth of the furnace lay six or eight feet below the brick flooring, and the effect of standing inside, with the fourteen round blowpipe holes admitting a little sunlight, was like being in a round ship's cabin, with fourteen portholes, except that the hollow furnace shot up to dark distances that the light didn't penetrate. ”

“ The gas on leaving the producer passes along fire-brick flues or passages to the furnace proper; the path which it is now caused to take varies somewhat according to the arrangement of the furnace in question. Modern gas-fired furnaces usually belong to one of two distinct types according to the manner in which the heat of the escaping products of combustion is utilised for heating the incoming gas and air ”

“ This simple drying of the blast saves about 19 per cent. of the fuel, and makes the action of the furnace much more regular than when ordinary air is used. It lowers the temperature of the gases which escape from the top of the furnace, and raises their percentage of carbon dioxide, symptoms of the great increase in fuel efficiency. Atmospheric moisture has a cooling effect on the lower part of a furnace, just where the highest temperature is needed to melt the iron and slag, remove the sulphur and deoxidize the silica. ”

“ Each is charged with its covered pot full of blistered steel with coal to match. Their lidded mouths dull the roaring sound of the terrible combustion, but the furnace-men show by their looks the intensity of the heat. The pouring-off sight is really thrilling. When the lid is removed from each furnace, and the pot of molten metal lifted out by a pair of long-handled tongs with rounded jaws, even a spectator must have steady nerves to look at it. ”

“ In short the electric furnaces can be used to improve the molten product of the Bessemer converter and open-hearth furnace, essentially because their atmosphere is free from sulphur and oxygen, and because they can therefore remove sulphur, iron oxide and mechanically suspended slag, more thoroughly than is possible in these older furnaces. They make a better though a dearer steel. ”

“ Small articles are readily fired in a Fletcher's crucible furnace supplied with a gas blow-pipe; the furnace is heated gradually to begin with. When a dull red heat is attained, the full power of the blast may be turned on, and the furnace kept at its maximum temperature for three or four hours at least, though on an emergency shorter periods may be made to do. ”

“ The air previously passes through a series of cast-iron pipes, heated by the combustion of the carbonic oxide obtained from the upper parts of the furnace, where it is formed as in a ‘gas-producer.’ The blast furnace acts continuously until it is worn out; the iron is tapped off twice a day, and the furnace is allowed to cool a little from time to time so as not to be spoilt by the increasing heat, and to enable it to withstand long usage. ”

“ As the iron melts it runs out through a tap hole and spout at the bottom of the furnace, to be poured into the moulds by means of clay-lined ladles. The air furnace is a reverberatory furnace like that used for puddling (fig. 14) , but larger, and in it the pigs of iron, lying on the bottom or hearth, are melted down by the flame from the coal which burns in the firebox. The iron is then held molten till it has grown hot enough for casting and till enough of its carbon has been burnt away to leave just the carbon-content desired, and it is then tapped out and poured into the moulds. ”

“ The following may aid in this duty:
When a furnace does not draw well, it often is owing to the stoppage by fine ashes or soot, and then the smoke-flues must be cleaned. The fewer and more simple the smoke-flues the less this trouble will occur. Sometimes the shaking of a furnace makes cracks in joints, and this causes outflow of gas and also diminishes the draught.
When iron is very hot, it burns the particles floating in the air, making an unpleasant smell and dryness. A large furnace, therefore, is better than a small one that must be kept very hot. ”

“ The burners should be so located that the flames from individual burners do not interfere nor impinge to any extent on the side walls of the furnace, an even distribution of heat being secured in this manner. ”

“ By my own experiments I have found, that the most powerful glass furnaces is only a weak fire, compared with that of bellows furnaces; and that fire produced in the focus of a good mirror, is stronger than that of the most glowing fire of a furnace. I have kept iron ore for thirty-six hours in the hottest part of the glass furnace of Rouelle, in Burgundy, without its being melted, agglutinated, or even in any manner changed ”

“ This increase in furnace volume in the direction of the flame insures free expansion and a thorough mixture of the oil with the air, and the consequent complete combustion of the gases before they come into contact with the tube heating surfaces. In such a furnace flat flame burners should be used, preferably of the Peabody type, in which the flame spreads outward toward the sides in the form of a fan. There is no tendency of the flames to impinge directly on the heating surfaces, and the furnace can handle any quantity of flame without danger of tube difficulties. ”

“ In the exterior wall, near its bottom, and opening on the lead flat, is a furnace of the commonest possible construction, and capable, from its free draught, of burning the commonest combustible material—such as inferior coal, coal screenings, coke, cinders, and sawdust. This furnace, encased in brickwork, is carried obliquely for a distance of four or five feet into the Calidarium; and its flue, following the angle of the floor, makes the circuit of the apartment. ”

“ Two years ago No. 1 broke out and the molten metal caught and burned to death one of our men and injured several others. ‘Hanging’ does not occur when the furnace is working right, but failure of the charge to come down evenly is very serious sometimes. A blast furnace is like a coquette ”

“ The hotter he can let the gas through, the quicker the brew is cooked, and the more "tonnage" he'll make that week.
"Get me thirty thousand pounds," said the first-helper when I was on the furnace that first night. Fifteen tons of molten metal! I was undecided whether to bring it in a dipper or in my hat. But it's no more than running upstairs for a handkerchief in the bureau. You climb to a platform near the blower, where the stuff is made, and find a man there with a book. ”

“ In these furnaces, the regenerator chambers themselves are generally placed underneath the melting furnace, and they are built of fire-brick and filled with loosely-stacked fire-bricks, whose function it is to absorb or deliver the heat. In the most modern type of furnaces of this class, the gas-regenerators are omitted entirely, the air only being pre-heated by means of regenerators, while the gas enters the furnace direct from the producer, thus carrying with it the heat generated in the producer during the gasification of the fuel. ”